Temperatures for thermoforming Polycarbonate

When Polycarbonate is cooled below 150 C / 302 F, it transitions from a flexible structure to a rigid structure that locks into what ever shape it is in; this temperature is known as the glass transition temperature. Conversely, when Polycarbonate it heated above its glass transition temperature it becomes flexible and can be bent into various shapes. This property is used in the process of thermoforming.

Thermoforming can be carried out at any temperature above the glass transition temperature and below the melt temperature of 267 C / 512 F, although in practice the Polycarbonate becomes more flexible the higher the temperature and it is not necessary to approach the melt temperature. The Polycarbonate actually becomes difficult to use much above a temperature of 215 C / 450 F.

There are three broad categories of forming – Cold forming, Low temperature thermoforming and high temperature thermoforming.

Cold forming.

Cold forming uses a frame to hold the Polycarbonate sheet in the desired shape. The sheet is then heated to between 302 F and 340 F for several hours until the entire sheet (interior and not just the surface) rises above the glass transition temperature. The sheet is then cooled below the glass transition temperature to set the shape. Cold forming is a simple process, but can only be used for relatively simple shapes (often two dimensional) without tight radius bends.

Low temperature thermoforming.

Low temperature thermoforming is carried out between 350 F and 370 F. This process is often used for simple shapes where the Polycarbonate sheet drapes over a mold or into a mold. While it is possible to achieve relatively simple 3D shapes with low temperature thermoforming, complex shapes with lots of detail are not possible. One advantage of low temperature thermoforming is that pre-drying of the sheets is not necessary.

High temperature thermoforming.

High temperature thermoforming is carried out between 370 F and 420 F. Complex shapes, sharp details and deep draws are all possible with high temperature thermo-forming. Many thermoforming processes use vacuum to achieve some of the complex shapes. One of the disadvantages of high temperature thermoforming is that all moisture must be removed from the sheet by drying the sheet prior to thermoforming. If this drying is not done, the higher temperatures will cause moisture evaporation bubbles to appear in the sheet during thermoforming.

Drying needs to be carried out above the boiling point of water and it is recommended that the sheet is heated to 120 C / 250 F to dry the material. The drying time is dependent upon the sheet thickness. For 0.118” thick sheet about 10 hours of drying is recommended, for 0.236” sheet, this can increase to closer to 30 hours. After drying the sheet should be used within a reasonably short time frame to prevent the sheet re-absorbing moisture from the air.

Hard coatings.

One thing to remember with thermoforming Polycarbonate sheet is that raising the temperature above the glass transition temperature will make the sheet flexible; any hard coating on the sheet will probably not be flexible and will crack during the thermoforming process. When purchasing Polycarbonate sheet for thermoforming it is important to use only hard coatings designed for thermoforming. These coatings are slightly more expensive than standard hard coats, but are considerably cheaper than the alternative of post coating any thermoformed parts.

Polycarbonate sheet - Temperature stability

When sputtering materials onto Polycarbonate sheets it is important to know what temperature the sheet can withstand. The sputtering process imparts energy to the sheet, which causes the temperature of the sheet to rise. The amount of temperature rise is dependent upon both the length of time and the amount of energy used in the sputtering process.

For example, laying down a relatively thin surface of ITO to give a surface resistance of 200 Ohms/square requires less time than laying down a thicker surface to give a surface resistance of 10 Ohms/square. The sheet will therefore reach a higher temperature when producing a 10 Ohms/square coating, all other things being equal.

This heating of the sheet is why it is relatively easily to put ITO onto glass, as it is not affected by temperature. It is also why it is much more difficult to put ITO onto acrylic than it is to put it onto Polycarbonate, because Polycarbonate is much more resistant to temperature than acrylic.

The ability of plastics to withstand temperature is typically reported on data sheets as the “Heat distortion temperature” as measured by either ISO 75 or ASTM D648. These tests measure the temperature at which deformation of the sheet first occurs when subject to a load. A typical reported result for Polycarbonate sheet would be:

“Heat distortion temperature under a load of 65 psi – 288 F / 142 C
Heat distortion temperature under a load of 260 psi – 264 F / 129 C”

As can be seen, the higher the load, the lower the temperature that the sheet deforms at.

These results, while useful for more traditional processing such as forming sheets, are not really relevant for sputtering type applications for two reasons:

- During a sputtering process the sheets are not subjected to a load but are usually located vertically in a rack.
- When sputtering we are not only interested in deformation of the sheet but, more importantly, in preventing a degradation of the optical properties.

Most Polycarbonate sheet available on the market has not been designed for high optical sputtering type applications. Instead they have been designed for commodity applications such as architectural glazing; in these applications they are not subjected to high temperatures. When these types of Polycarbonate sheet are heated by vacuum coating, some of the inherent stresses in the sheet can lead to optical distortion. This distortion is often unacceptable in optical applications.

HighLine Polycarbonate has specifically designed a range of Polycarbonate sheet for vacuum deposition applications. The material is formulated to be resistant to higher temperatures, has the required optical properties and has reduced internal stresses in the sheet. In a future blog we will discuss how these internal stresses can be both visualized and measured in both commodity Polycarbonate sheet and our advanced materials.

For vacuum deposition processing we have found that our sheet can easily withstand continuous temperatures of 145 C / 293 F for over three hours without any loss in optical performance or distortion of the sheet.